Why a 2,500-ton light cruiser is effectively a destroyer

How useful would Japanese 2,200- to 2,500-ton light cruisers based on the Ning Hai class have been?

Not very useful as cruisers; at that displacement they would effectively be destroyers.

The concept fails primarily on displacement. At roughly 2,200 to 2,500 tons, the proposed ships sit in destroyer territory regardless of their light-cruiser label. Their small size would sharply restrict the capabilities expected from a cruiser. Classification cannot substitute for hull volume and usable capacity.